1. Field of the Invention
The invention relates to the field of wireless communications, more particularly to a method of and device for switching between antennae communicating with a diversity receiver, each of the antennae receiving signals transmitted from a single source.
2. Description of the Related Prior Art
As will be appreciated by those skilled in the art, within a wireless communication system, maintaining sufficient signal strength to provide for continuous communication between parties is critical. A common problem in wireless communications is interference caused by nearby transmissions on the same or adjacent frequency bands. Interference can cause a receiver to misinterpret a transmitted signal, become jammed, fail to correlate a received signal, or can have other adverse effects on the ability of a receiver to properly receive transmitted information. In addition, receivers in a wireless communication system can also suffer from multipath and fading, which can cause the received signal to fluctuate in amplitude, phase or other characteristics in a relatively short period of time.
Avoiding “dropouts” or “nulls” caused by multipath phase cancellations has been the focus of much attention in the wireless industry. Dropouts occur when the transmitter and receiver antennas are in a particular location relative to one another. As shown in FIG. 1, the signal from the transmitter 10 reaches antenna A of receiver 12 via a direct path and a reflected path. The reflected signal path is a bit longer than the direct path, causing the two signals to be put out of phase when they mix together at the receiver antenna A. The resulting weak signal causes a dropout. Moving the transmitter 10 or receiver 12 to a different location may reduce or eliminate dropouts. However, sometimes relocating these devices is not practical or possible. As a result antenna diversity techniques were developed to overcome dropouts.
The basic concept of antenna diversity is depicted in FIG. 2. As shown in the figure, two antennas A and B are used. The signal arriving from transmitter 10 at antenna A is largely cancelled by a multipath null, leaving little signal left for the diversity receiver 14. However, the signal at antenna B remains strong and provides adequate signal for the receiver 12 to produce a usable audio signal to noise ratio. Generally speaking, the spacing between antennas A and B must be at least ½ wavelength of the operating frequency to ensure that antennas A and B are receiving uncorrelated (i.e. “Diverse”) signals to gain the full benefit of diversity reception.
As shown in FIG. 2, the signals received from antennas A and B are processed in a diversity receiver 14. Diversity circuitry implemented in a high quality receiver with excellent sensitivity will reduce or eliminate multipath dropouts and, in some cases, increase operating range. The improvement in reception will vary depending upon the diversity methodology chosen by the designer. The type of diversity reception circuitry chosen in the receiver design includes a number of considerations, including cost, size and weight, performance and the practicality of each circuit type for a given application. The manner in which incoming signals from two different antennas are handled after they enter the receiver is what makes the difference between a highly efficient and a less efficient receiver. Using diversity reception makes little sense unless the receiver is a high quality design to begin with.
It will be also understood by those in the art that there are several different techniques used for diversity reception in various designs with varying degrees of success. These techniques can be broadly divided into two groups: non-active; and active. As shown in FIG. 3A, passive diversity is a non-active technique which involves the addition of a second antenna to a single receiver, placed ½ wavelength or more away. The antennas A and B are connected to a combiner 16 which adds the two signals. However, when the received signals are out of phase with one another dropouts will occur.
As shown in FIG. 3B, active diversity reception techniques include: antenna phase switching diversity; audio switching diversity; and ratio diversity. With antenna phase switching diversity, two antennas A and B are mixed to feed a single combiner 16, with a phase reversal switch (not shown) added to the input of one of the antennae. When signal conditions deteriorate, the phase of one of the antennas is reversed and logic circuitry 18 then determines whether or not the switching action has improved the signal to noise ratio, and decides whether to latch in that position or switch and sample again. The problem with this solution is that the receiver doesn't react until it is already in trouble. Further, there is always the possibility that switching phase will make a marginal problem worse. Finally, since the switching circuitry is in the RF signal path and is triggered only at RF levels, it can produce a “click” when the switching occurs.
As shown in FIG. 3C, audio switching diversity uses two receivers 20, 22, selecting the audio output of one of the receivers. The switching action is usually triggered by comparing incoming RF levels and switching to the receiver with the stronger RF signal, which usually produces a better signal to noise ratio in multipath conditions. The downside of this solution is that two receivers are required, meaning that the physical size and power requirements will be greater. In order to implement this solution in a compact, battery powered receiver, serious compromises must be made in the circuit design to reduce the physical size and power requirements.
Finally, As shown in FIG. 3D ratio diversity utilizes two separate receivers 20, 22, sharing a common oscillator and audio circuitry. The audio inputs of the receivers are used simultaneously, being mixed by a “panning” circuit 24 in a ratio controlled by the comparative RF levels at the receivers 20, 22. This method anticipates dropouts long before they occur, since the comparative RF level sampling and mixing starts taking place at higher RF signal levels than in other diversity designs. By the time the signal level at one receiver drops low enough to produce noise, the panning circuitry 24 has long since shifted to the other receiver. Unlike audio switching diversity, a ratio diversity receiver utilizes both receivers simultaneously. When the overall signal is low and the receiver is struggling to find enough signal to produce a useable signal to noise ratio, the ratio diversity receiver will continue to balance the outputs of the two receivers for the lowest noise, even at very low RF levels. Unfortunately, like the audio switching diversity arrangement, two receivers are required, resulting in the same disadvantages.
As will also be appreciated by those in the art, diversity receivers select the antenna to be used for each individual burst based on a metric that is measured during the beginning of each received burst or packet. Generally speaking, quick, precise measurement on a per packet basis is desirable to ensure that the best possible signal is being processed. Fast diversity decisions require a low latency filter, but such a filter may not reject noise and interference very well. This is a tradeoff decision impacting the performance of the diversity algorithm. Allowing for high latency enables the design of a filter that has better noise to interference rejection which is beneficial during data transmission after the header where latency is less of a concern.
The metric upon which the antenna selection is based can either be a signal quality index or a signal power index. Signal quality indices will often use a matched filter to generate the decision metric. However, the timing budget may not allow for a reliable signal quality index to be generated for all antennae. The signal power index is a preferred alternative since it takes less time to measure. However, the use of the signal power index in has not been efficiently incorporated into a diversity selection receiver to date.
In light of the problems and deficiencies highlighted above, there is a need, therefore, for an improved diversity receiver capable of effectively preventing a deterioration in transmission efficiency.